Christoph Rueger created FREEMARKER-213:
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Summary: Handling AutoCloseable Iterators in ?filter / ?first when
the iterator is not fully consumed
Key: FREEMARKER-213
URL: https://issues.apache.org/jira/browse/FREEMARKER-213
Project: Apache Freemarker
Issue Type: Improvement
Components: engine
Affects Versions: 2.3.31
Reporter: Christoph Rueger
HI,
We have expressions like:
{code:java}
${myCatalog.getCSVFileRowsIterator()?filter(row -> row.get("categoryId") ==
"123")?first}
{code}
Unter the hood this becomes an IteratorModel().
The problem is that:
- the underlying iterator is working on a resource (file, stream)
- which needs to be *closed when finished*.
- but ?filter and ?first can abandon and leave the iterator somewhere in the
middle once it has identified a match
- this caused Resource Leaks in some cases.
I debugged down to the following code in _freemarker.core.new
TemplateModelIterator() {...}.ensurePrefetchDone())_
{code:java}
private void ensurePrefetchDone() throws TemplateModelException {
if (prefetchDone) {
return;
}
boolean conclusionReached = false;
do {
if (lhoIterator.hasNext()) {
TemplateModel element =
lhoIterator.next();
boolean elementMatched;
try {
elementMatched =
elementMatches(element, elementTransformer, env);
} catch (TemplateException e) {
throw new
_TemplateModelException(e, env, "Failed to transform element");
}
if (elementMatched) {
prefetchedElement = element;
conclusionReached = true;
}
} else {
prefetchedEndOfIterator = true;
prefetchedElement = null;
conclusionReached = true;
}
} while (!conclusionReached);
prefetchDone = true;
}
{code}
It looks like at this place we have the "iterator" and also know when we stop
iterating, when finding a match.
*Question*
Would it be possible to check if the underlying Iterator implements
_java.lang.AutoCloseable_ and if yes, call _.close()_ on it, when a match is
found?
I am not sure if that is the right thing to do here.
We have built workarounds so that AutoCloseable objects in the datamodel are
closed after rendering is finished.
But it would be better to close it as soon as possible, when the iterator is
stopped to be consumed. This would help in scenarios where the template does
that multiple times, e.g.:
{code:java}
${myCustomObject.getCSVRowsIterator()?filter(row -> row.get("categoryId") ==
"123")?first}
${myCustomObject.getCSVRowsIterator()?filter(row -> row.get("categoryId") ==
"234")?first}
{code}
Ideas would be welcome, where this kind of "closing" should happen.
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